Data from the Vaginal Infections and Prematurity (VIP) Study were analyzed to determine the relationship between the frequency of sexual intercourse during pregnancy and preterm delivery and how this relationship is affected by vaginal microbiology. The population consisted of women seeking prenatal care from 23 to 26 completed weeks of pregnancy at one of seven clinical sites (Columbia University and Harlem Hospital, New York; University of Washington, Seattle; University of Oklahoma, Oklahoma City; University of Texas, San Antonio; Tulane and Louisiana State Universities, New Orleans) from 1984 to 1989. The women were enrolled in a standardized protocol and upon entry (23-26 weeks' gestation) were interviewed to obtain demographic, social, and sexual histories and underwent physical examinations, at which time a standardized set of genital culture specimens were obtained. The association between frequency of sexual intercourse and preterm birth was studied among women with and without various genital microorganisms.